Present day commercial mobile communication systems typically have a plurality of fixed-location base stations arranged in patterns in which each base station has a plurality of antennae divided among a number of sectors, to transmit and receive on a number of frequencies. The distribution of antennae, sectors and assignment of frequencies as well as the selection of antennae is intended to be optimize coverage. The channels used by a base station are separated from one another in frequency sufficiently that signals on any channel do not interfere with signals on another channel from that base station.
However, in order to allow mobile units to transmit and receive telephone communications over a wide area, each cell is normally positioned with its area of coverage adjacent and overlapping the areas of coverage of a number of other cells. When a mobile unit moves from an area covered by one base station to that covered by another, the communication is transferred from the former station to the latter in an area where the coverage from the different cells overlaps. Because of this overlapping coverage, the channels allotted to the cells are carefully selected so that adjoining cells do not transmit or receive on the same frequencies. The channels used by adjoining base stations are also supposedly separated from one another in frequency sufficiently that signals from any base station do not interfere with signals from another nearby base station. This is typically accomplished by assigning channels to some central cell, which are widely separated in frequency in the manner described above, and then assigning channels to the cells surrounding that central cell using a pattern to optimize coverage and minimize interference. The pattern of channel assignments continues similarly in the other cells adjoining the central cell. It will be noted that the technique of separating each channel assigned to any cell from the next channel assigned to that cell allows a number of cells having entirely different frequencies to be positioned in a system before any frequency must be repeated. The pattern is often called a frequency reuse pattern and may vary in many ways.
In some systems, especially those with cells in urban areas carrying heavy traffic, each cell is further divided into three sectors each of which includes assignment of channels. The antennas of each sector are typically arranged to provide 120 degrees of coverage within the cell. With slightly over four hundred channels available, a repeating pattern may be achieved in a hexagonal arrangement, using seven cells that each have three sectors.
In theory, this form of cell arrangement and the channel assignment scheme allow the frequency reuse pattern to be repeated at distances sufficiently separated to minimize interference between mobile units.
Unfortunately, interference does occur for a number of reasons. Antenna patterns, power levels, scattering, and wave diffraction differ from cell to cell. Buildings, hills, mountains, foliage, and other physical objects cause the transmitted signal strength to vary over the region covered by a cell. Consequently, the boundaries at which the signal strength of a channel falls below a level sufficient to support communications with a mobile unit vary widely from cell to cell. For this reason, cells adjacent to one another do not, in fact, typically form the precise geometric boundaries suggested above and coverage gaps can occur. Since cell boundaries must overlap to provide complete coverage of an area and the boundaries of cells are imprecisely defined, overlapping coverage often occurs between channels. As a result, channels having an overlapping coverage area may interfere with each other.
The odd-shaped boundaries of the cells and the necessity that the cells have overlapping coverage areas increases the likelihood that multiple signals on the same channel will interfere with each other, even though they are generated by non-adjacent or widely separated cells. This is especially true when a sectored cell pattern is used because the cells are much closer to each other than in a simple cell pattern. A signal originating from a remote cell (sometimes referred to as an interferor I) may interfere with a second stronger signal carrying a mobile transmission (sometimes referred to as the carrier signal S), on the same channel, when the difference in signal strength between the two signals is less than some threshold level (typically measured in decibels as the S/I ratio). Moreover, signals on adjacent channels are carried by abutting cells in accordance with the above-described frequency reuse pattern. Typically, frequency filtering is insufficient to eliminate the reception of adjacent frequencies entirely. Consequently, there may be adjacent channel interference. A signal on an adjacent frequency may interfere with a communication link when the difference in signal strength between the two signals is less than some second, usually lower, threshold level. Normally an adjacent channel may have a signal strength closer to that of the communication link without causing significant interference because the frequency filtering of the receivers eliminates a substantial portion of the adjacent channel's signal strength.
In order to optimize coverage and to overcome interference when designing or reconfiguring the coverage of a mobile cellular system, a cellular system operator uses predictive software to determine what signal strength may be expected at locations throughout the cellular system from each of a particular set of cells. This software can utilize data describing the physical characteristics of the terrain surrounding each cellular site and the physical characteristics of the cellular station to plot predicted signal strengths around the cellular site. These signal strength predictions are then overlaid on a graphical plot to determine where antennas should be placed to provide optimum coverage, with appropriate overlapping areas for hand-offs. Once the antenna sites have been determined, the operator assigns channel groups to the cells in accordance with the technique described above.